This invention relates to the Ionized Physical Vapor Deposition (IPVD) and, more particularly, to methods and apparatus for depositing films, most particularly metal films, onto semiconductor wafer substrates by sputtering the coating material from a target, ionizing the sputtered material, and directing the ionized coating material onto the surface of the substrates.
Ionized physical vapor deposition is a process which has particular utility in filling and lining high aspect ratio structures on silicon wafers. In IPVD, for deposition of thin coatings on semiconductor wafers, materials to be deposited are sputtered or otherwise vaporized from a source and then a substantial fraction of the vaporized material is converted to positive ions before reaching the wafer to be coated. This ionization is accomplished by a high-density plasma which is generated in a process gas in a vacuum chamber. The plasma may be generated by magnetically coupling RF energy through an RF powered excitation coil into the vacuum of the processing chamber. The plasma so generated is concentrated in a region between the source and the wafer. Then electromagnetic forces are applied to the positive ions of coating material, such as by applying a negative bias on the wafer. Such a negative bias may either arise with the wafer electrically isolated, by reason of the immersion of the wafer in a plasma, or by the application of an RF voltage to the wafer. The bias causes ions of coating material to be accelerated toward the wafer so that an increased fraction of the coating material deposits onto the wafer at angles approximately normal to the wafer. This allows deposition of metal over wafer topography including in deep and narrow holes and trenches on the wafer surface, providing good coverage of the bottom and sidewalls of such topography.
Certain systems proposed by the assignee of the present application are disclosed in U.S. patent applications Ser. Nos. 08/844,751; 08/837,551 and 08/844,756 filed Apr. 21, 1997, hereby expressly incorporated herein by reference. Such systems include a vacuum chamber which is typically cylindrical in shape and provided with part of its curved outer wall formed of a dielectric material or window. A helical electrically conducting coil is disposed outside the dielectric window and around and concentric with the chamber, with the axial extent of the coil being a significant part of the axial extent of the dielectric wall. In operation, the coil is energized from a supply of RF power through a suitable matching system. The dielectric window allows the energy from the coil to be coupled into the chamber while isolating the coil from direct contact with the plasma. The window is protected from metal coating material deposition by an arrangement of shields, typically formed of metal, which are capable of passing RF magnetic fields into the interior region of the chamber, while preventing deposition of metal onto the dielectric window that would tend to form conducting paths for circulating currents generated by these magnetic fields. Such currents are undesirable because they lead to ohmic heating and to reduction of the magnetic coupling of plasma excitation energy from the coils to the plasma. The purpose of this excitation energy is to generate high-density plasma in the interior region of the chamber. A reduction of coupling causes plasma densities to be reduced and process results to deteriorate.
In such IPVD systems, material is, for example, sputtered from a target, which is charged negatively with respect to the plasma, usually by means of a DC power supply. The target is often of a planar magnetron design incorporating a magnetic circuit or other magnet structure which confines a plasma over the target for sputtering the target. The material arrives at a wafer supported on a wafer support or table to which RF bias is typically applied by means of an RF power supply and matching network.
A somewhat different geometry employs a plasma generated by a coil placed internal to the vacuum chamber. Such a system does not require dielectric chamber walls nor special shields to protect the dielectric walls. Such a system is described by Barnes et al. in U.S. Pat. No. 5,178,739, expressly incorporated by reference herein. Systems with coils outside of the chamber, as well as the system disclosed in the Barnes et al. patent, involve the use of inductive coils or other coupling elements, either inside or external to the vacuum, that are physically positioned and occupy space between the planes of the sputtering target and the wafer.
Whether a coupling element such as a coil is provided inside or outside of a vacuum chamber, dimensions of the system have been constrained by the need for adequate source-to-substrate separation to allow for the installation of the RF energy coupling elements between the source and the substrate. Adequate diameter must also be available around the wafer for installation of coils or other coupling elements. As a direct result of the increased source-to-substrate spacing due to the need to allow space for the coupling element, it is difficult to achieve adequate uniformity of deposition with such systems. If the height of the chamber is reduced to improve uniformity, there is a loss of plasma density in the central region of the chamber and the percentage of ionization of the coating material is reduced. Also, in practice, the entire system must fit within a constrained radius. As a result, there are frequently problems due to heating arising from the proximity of the RF coils to metal surfaces, which may necessitate extra cooling, which increases engineering and production costs and wastes power.
An IPVD apparatus with the coil in the chamber has the additional disadvantage that the coils are eroded by the plasma and must, therefore, consist of target grade material of the same type as that being sputtered from the target. Moreover, considerable cooling of coils placed in the vacuum chamber is needed. If liquid is used for this cooling of the coils, there is danger that the coils will be penetrated, by uneven erosion or by arcing, causing a resulting leak of liquid into the system, which is highly undesirable and will likely result in a long period of cleaning and re-qualification of the system. Furthermore, an excitation coil in the chamber also couples capacitively to the plasma, leading to inefficient use of the excitation power and to the broadening of the ion energy spectrum, which may have undesirable effects on the process.
The miniaturization of semiconductor devices has resulted in a need to form low resistance connections to contacts at the bottoms of high aspect ratio holes of a fraction of a micron in diameter. This has increased the demand for the use of highly electrically conductive metals, such as copper, over barrier layers of materials, such as tantalum and tantalum nitride. The techniques for depositing such materials in the prior art have not been totally satisfactory.
The deposition of materials by PVD methods has, in the prior art, involved critical designs of sputtering sources to produce plasma concentrations of uniform geometries within sputtering chambers and to directly affect the distribution uniformities of the deposited films. The prior art approaches have resulted in compromises of other performance parameters to those ends.
As a result of the above considerations and problems, there remains a need for more efficiently coupling energy into the dense coating material ionizing plasma in IPVD processing systems, and to do so without interfering with the optimum dimensions of the chamber and preferably without placing a coil or other coupling element into the vacuum chamber.
One objective of the present invention is to provide an IPVD method and an IPVD apparatus in which the placement of the coil or other coupling element does not adversely affect the geometry of the chamber of the processing apparatus. Another objective of the present invention is to provide a more efficient and effective method and apparatus for the performance of IPVD.
According to the principles of the present invention, an IPVD apparatus is provided with a ring-shaped source of coating material for producing a vapor that includes atoms or minute particles of the coating material to a processing space within a vacuum chamber. At the center of the ring-shaped source is provided a coupling element for reactively coupling RF energy into the chamber to produce a high-density, reactively coupled plasma in the processing space to ionize coating material passing through the processing space. The ions of coating material drift, whether under the influence of electrostatic or electromagnetic fields or otherwise, toward a substrate in the chamber, at the opposite end of the processing space from the source. Those ions that arrive within a certain distance, for example, in the order of a centimeter, from the substrate, encounter a sheath field and are accelerated toward the substrate so that a high percentage of the coating material arrives on the substrate at angles normal to the substrate, thereby more effectively lining the bottoms and sides of, or filling, small and high aspect ratio features on the surface of the substrate.
In one embodiment of the invention, a coating material source, preferably a sputtering target, is provided with a central opening in which is placed a dielectric window. Behind the window, outside the vacuum of the chamber, is located a plasma source which includes a coupling element, preferably a coil, which is connected to the output of an RF energy source. The coupling element is configured to couple, preferably inductively, energy supplied from the energy source through the window at the opening at the center of the material source and into the region of the chamber between the coating material source and the substrate, such as a semiconductor wafer, on a substrate support at the opposite end of the chamber from the coating material source.
The apparatus of the present invention includes an annular sputtering target which surrounds a central ceramic window. This annular target is preferably frusto conical in shape. A magnetron magnet assembly is positioned behind the target to produce a plasma confining magnetic field over the target, preferably in the shape of an annular tunnel on the surface of the annular target surrounding the central opening at its center.
The coupling element is preferably a coil positioned behind and close to the back outside surface of the dielectric window at the central opening of an annular sputtering target. RF energy of, for example, 13.56 MHZ, is applied to the coil to excite a high-density inductively coupled plasma in the chamber between the target and the substrate. A main sputtering plasma that is trapped under the field of the magnetron magnets at the surface of the target, sputters coating material from the target and into the region of the processing space occupied by the dense secondary plasma, where a substantial portion of the material is stripped of electrons to form positive ions of the coating material. A negative bias voltage is applied to a wafer on the substrate holder, which attracts the positive ions of sputtering material from the region of the secondary plasma and toward and onto the surface of the substrate, with the angles of incidence approaching being perpendicular to the substrate so that they can enter trenches and holes on the wafer substrate to coat the bottoms of these holes and trenches.
Certain embodiments of the apparatus and method of the invention include an IPVD source that employs a three-dimensional coil that energizes a dense inductively coupled three-dimensional plasma in three-dimensional regions within the chamber. The chamber is operated at a vacuum pressure of between 30 and 130 mTorr to essentially thermalize the plasma, so that ions of coating material can be formed in the plasma and electrically directed perpendicular to and onto the substrate, thereby reducing the effect of target and magnet configuration on coating uniformity. The IPVD source is coupled through a window into the chamber through a high dielectric material such as a TEFLON spacer and then through a dielectric window such as quartz which forms the vacuum barrier closing a circular opening in the chamber wall at the center of an annular target. Inside the chamber is a window shield having chevron-shaped slots therein oriented relative to the conductors of the coil. The shield protects the window from deposits, particularly deposits of metallic coating material, while passing inductively coupled RF energy into the chamber. The shield may further function as a Faraday shield, preventing capacitive coupling from the coil to the plasma and avoiding flux compression heating. The shield has integral cooling and is formed of cast copper which is plated with aluminum, so that the shield can be reconditioned by chemically dissolving aluminum coating to remove buildup and then re-plating the copper shield with aluminum for reuse. The window and shield assembly form a removeable combination. The window and shield are spaced so the window is self cleaning adjacent the slits in the shield by plasma that forms at this point in the slits.
The target is preferably frusto conical, with the walls of the truncated cone inclined about 35xc2x0 to the horizontal or plane of the window. A permanent magnet pack is employed which produces three, and preferably only three, magnetic tunnels over the target surface, with a main central tunnel dominating early in the target life to erode the mean radius of the annular target and two side tunnels taking over later in the life of the target to erode grooves adjacent the inner and outer rims of the target annulus.
The apparatus preferably uses a wafer holder mounted for vertical motion on a Z-table motion drive to provide for target-to-substrate spacing (TSS) of from six to nine inches and to provide for wafer handoff to a transfer arm from a transfer module. The support is provided with an electrostatic chuck, and wafer heating and cooling is provided using a Peltier device remote from the support that connects through a GALDEN fluid loop with the support and through another fluid loop with a heat sink. The electrostatic chuck is tri-polar with the chuck grid serving as electrodes to provide 2-zone bias to the wafer to attract the ionized sputtered material to the wafer. A shadow ring is provided around the edge of the wafer to provide non-contact edge masking.
The chamber has a removable shield insert in two parts that mechanically float relative to each other to accommodate different expansions due to different heating. The shield assembly is a replaceable subcombination. The apparatus is particularly useful for depositing copper over tantalum and tantalum nitride and for depositing the underlying tantalum and tantalum nitride barrier layer over a patterned wafer, with Ta deposited by ionized PVD and TaN deposited by PVD in the same chamber, followed by deposition of copper by ionized PVD in a similar module attached to a transfer module of the same tool. The copper so deposited is suitable to be followed by any of many methods of Cu fill, particularly by electroplating. The processes are preferably carried out using process parameters, including: pressures, temperatures, gases, bias power and/or voltage levels, sputtering power levels, IC power levels, etc., as described below.
With apparatus structure according to the invention, the processing chamber can be dimensioned to provide optimum spacing between the coating material source and the substrate to provide both good ionization of sputtered species as well as good uniformity of deposition on the wafers.
The present invention provides greater freedom of design choice in configuring the processing chamber to optimize the IPVD process, and does so while overcoming the difficulties set forth in the background above.